The three IARU regional organizations develop band plans to offer guidance to radio amateurs on how the different operating interests and modes of emission can be best accommodated in the limited spectrum available. Observing band plans is good amateur operating practice. In most cases observance is voluntary although some administrations incorporate the band plan for their region into national regulations. Also, many contest sponsors require that competitors adhere to band plans.
Of course, if a band plan conflicts with national regulations the national regulations must be observed.